There are many situations in which the level of solid particles within a vessel (including bins or pipes) is desirably determined. In the pulp and paper production field there are many situations where the level of pulp, or wood chips during the production of pulp, need be accurately determined, such as in continuous wood chip digesters for the production of chemical paper pulp.
One useful prior art device for determining the level of solids within a vessel is shown in U.S. Pat. No. 3,971,254, the disclosure of which is hereby incorporated by reference herein. In that device, the torsional movement of a shaft disposed within the vessel and connected to a paddle (plate extending perpendicular to the flow of solids) is sensed utilizing strain gauges mounted on a thin walled portion of the shaft, interiorly of the shaft. Shear pins are provided connecting the shaft and the housing for the shaft so that they are the weakest portion of the assembly, breaking before the paddle or shaft will break. While the device illustrated in that patent is very useful and successful, it has some drawbacks associated with it. Typically the strain gauges are glued to the inside of the thin walled portion of the shaft. Gluing of the sensors is precision work for a skilled worker, not something that can be handled by the average maintenance technician. Also, the glue curing procedure requires control of the temperature and makes it difficult to replace a strain gauge without interrupting the operation of the vessel (e.g. digester) with which the level sensor is associated. Also, in the cooking zone of a digester or the like, it is often necessary to provide cooling water between the shaft and the housing for cooling the strain gauges to keep them operating properly.
According to the present invention, a level detector or the like, and a method of detecting solids within a vessel, are provided which overcome the above-mentioned problems. The invention utilizes components that may be mounted entirely exteriorly of the vessel so that they are readily accessed for replacement or repair. Preferably a differential transformer is provided, and is mounted by bolts directly on the outside of the vessel, so that it may be readily removed, replaced, or repaired. Also, no cooling water is necessary since the coils of the differential transformer can withstand at least 180.degree. C, and no glue is necessary for mounting. Further, the shear pins provided in the U.S. Pat. No. 3,971,254 indicator can be eliminated, as can a pressure seal that allows rotation of the shaft in the prior art device. This is so since the shaft whose torsional movement will be sensed can be made of greater diameter and wall thickness, while still sensing the twisting force thereon with great enough sensitivity to indicate the presence or absence of solids at that particular point within the vessel.
According to a first aspect of the present invention a detector is provided for detecting the solids position in a vessel which is adapted to have solid particles moving therein along an axis thereof. The detector comprises: A first hollow shaft having a shaft axis passing through the center thereof. A second shaft, generally concentric with the first shaft, and extending along the shaft axis and having first and second ends. Means for operatively fixing the first shaft at a first portion thereof to the vessel, with a first end remote from the first portion and extending into the vessel. Means for operatively connecting the second shaft first end to the first end of the first shaft, the second end of the second shaft being free. A plate operatively connected to the first shaft first end and the second shaft first end, so that it is generally perpendicular to the axis of the vessel. And, sensing means, including a movable element mounted to the second shaft second end, and a stationary element adjacent the movable element, for sensing twisting motion of the first shaft as a result of solid particles acting on the plate. The sensing means preferably comprises a differential transformer; i.e. a movable core element made of high magnetic permeability material, and a stationary core element also of such material. A plurality of coils are operatively associated with the core elements, and alternating current is provided to at least one of the coils. A voltage detector detects changes in voltage due to variations in the magnetic flux in the core elements, and an indicator is operatively connected to the detector. The indicator thus indicates the presence or absence of solids acting on the plate.
The invention also relates to a solids level detecting assembly which comprises a vessel adapted to have solid particles moving downwardly therein, having a generally vertical axis; means for responsive to the level of solid particles in the vessel, including the plate; and means for mounting the level responsive means in the vessel so that the plate is within the vessel and is disposed generally horizontally. The means responsive to the level of the solid particles preferably includes a movable core element of high magnetic permeability material, means for operatively connecting the plate to the movable core element, a stationary core element of high magnetic permeability material, a plurality of coils operatively associated with the core elements, means for providing AC current to at least one of the coils, a voltage detector for detecting changes in voltage due to variations in the magnetic flux in the core elements, and an indicator operatively connected to the detector.
The invention also contemplates a method of detecting the level of solids in a generally vertical axis vessel utilizing a movable core element of high magnetic permeability material that is pivotal with respect to a stationary core element of high magnetic permeability material to increase or decrease the gap between portions thereof, and a magnetic flux detector for detecting the changes in the magnetic flux due to movement of the movable element with respect to the stationary element. The method comprises the steps of: (a) Causing the solids to move generally downwardly within the vessel. (b) Disposing a physical element in the vessel at a given level to engage any solids in the vessel at that given level. (c) Transforming movement of the physical element in response to the presence or absence of solids in the vessel at that given level to pivotal movement of the movable core element with respect to the stationary core element. And, (d) sensing and indicating the change in magnetic flux in the core elements as a result of the pivotal movement of the movable element with respect to the stationary element. Step (b) is preferably practiced by disposing a flat plate perpendicular to the vessel axis, and the vessel, and step (c) is preferably practiced by mounting the plate to a shaft mounted for torsional movement.
It is the primary object of the present invention to provide a reliable solids level detector or the like that is simple and easy to utilize, repair, and replace. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.